Waveguide structure

ABSTRACT

A waveguide structure comprising: a core comprising a layer of electro-optic dielectric material, a first layer of semiconductor material provided below the electro-optic material and a second layer of the semiconductor material provided above the electro-optic material, and electrodes, configured for applying voltages. The electro-optic dielectric material has a Pockels tensor containing at least one non-vanishing element rij where i≠j, and the electrodes comprise a first set of electrodes provided substantially in direct contact with the electro-optic dielectric material, and a second set of electrodes comprising at least an electrode provided substantially in direct contact with the first layer and at least an electrode substantially in direct contact with the second layer, wherein the sets of electrodes are configurable to apply in the electro-optic material, at least a substantially horizontal electrical field and at least a substantially vertical electrical field that are orientated substantially perpendicular relative to each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority fromcommonly-owned United Kingdom Patent Application 1407260.7, filed onApr. 24, 2014.

FIELD OF THE INVENTION

The present invention relates to a waveguide structure and also extendsto a method of fabricating such a waveguide structure.

BACKGROUND OF THE INVENTION

For the integration of photonic circuits/structures into silicon,several building blocks such as modulators, waveguides and detectors areneeded. In order to link the electrical domain and the optical domain,devices such as electro-optic modulators need to be developed. In thefield of telecommunication, electro-optic materials such as lithiumniobate have been used to modulate light at relatively lower power andhigher speed. This mature technology has not yet been applicable insilicon-based photonics as silicon does not inherently show any linearelectro-optic effect. Also, electro-optic materials that may beintegrated into silicon-based photonic structures without the need forspecific processing techniques, such as, for example, spin-coating,and/or that may be compatible for the mass fabrication of such devicesare not yet generally available. The use of plasma dispersion effects insilicon may alleviate some of the issues limiting silicon-basedphotonics since materials in addition to silicon may not be needed.However, the exploitation of such effects does not enhance theperformance of silicon-based photonic modulators to the extent possiblewith modulators based on electro-optic materials, which provide anincreased bandwidth by facilitating higher order modulation schemes.

It is known that electro-optic active materials can be integrated inwaveguide structures that are then used to fabricate electro-opticalmodulators or switches, such as ring resonators and Mach-Zehndermodulators. An electro-optic active material is a material whoserefractive index can be varied by applying an electrical field to thismaterial, such an electrical field hereinafter being referred to as themodulating electrical field. Varying the refractive index of theelectro-optic active material via the modulating electrical field can beused to affect the passage of an optical signal/light traversing throughthe electro-optic active material. The electro-optic effect of anelectro-optic active material may depend on certain factors such ascrystal orientation, the electrical fields that are applied to theelectro-optic active material and also the orientation of light withrespect thereto. The extent to which the variation of the refractiveindex occurs for a given modulating electrical field comprises theelectro-optic response of an electro-optic active material.

In respect of electro-optic active materials, it is known that bariumtitanate has a relatively large associated electro-optic effect and sowould be desirable for the basis of an optical modulator. A polingelectrical field is applied to barium titanate to align/pole itsferroelectric domains. This is done to be able to record a macroscopicchange in the refractive index of the barium titanate when a modulatingelectrical field is applied thereto. The electro-optic response ofbarium titanate is increased when the modulating electrical field andpoling electrical field are orientated substantially perpendicular withrespect to each other.

Reference is now made to US2010/0111303A1 which describes anelectro-optic waveguide polarisation modulator comprising a waveguidecore having first and second faces defining a waveguide core plane, aplurality of primary electrodes arranged at a first side of thewaveguide core plane and out of said plane, and at least one secondaryelectrode arranged at a second side of the waveguide core plane and outof said plane, wherein the electrodes are adapted in use to provide anelectric field having field components in two substantiallyperpendicular directions within the waveguide core so as to modulate therefractive index thereof such that electromagnetic radiation propagatingthrough the core is converted from a first polarisation state to asecond polarisation state. This document discloses a wave-guide core andcladding provided on the core, which separates the core from two topelectrodes. The voltages applied to the two top electrodes are used tofacilitate an electrical field having a horizontal field component and avertical field component in the core, which are described as beingperpendicular to each other. Consideration is now made as to whether thedescribed configuration would be suitable for when the core compriseshigh-permittivity and/or electro-optic active materials, such as, forexample, barium titanate. Even though the electrical field componentsmay be perpendicular to each other in the present configuration, it isunlikely that the horizontal field component is present at the edges ofthe core. So, in respect of the core comprising barium titanate, therewould be no poling electrical field at the edges of the core and soferroelectric polarisation of the barium titanate in these regions isunlikely. Furthermore, a relatively large voltage drop occurs in thecladding for voltages applied to the two top electrodes and so thehorizontal and/or vertical electrical field components may haverelatively low associated field strengths in the core. Thus, theeffectiveness of the poling field and/or the modulating electrical fieldin the electro-optic active material when it comprises barium titanateis expected to be reduced as is its associated electro-optic response.

Turning to U.S. Pat. No. 4,691,984, this document discloses awavelength-independent electro-optical polarisation mode convertercomprising: an electro-optical crystal substrate cut in a plane definedby a direction perpendicular to the optical axis of the crystal; anoptical waveguide formed by diffusion of material into a surface of thesubstrate, the waveguide being oriented to provide for the propagationof light from one end to the other, in a direction parallel with theoptical axis of the substrate; and electrode means disposed on thesurface of the substrate and positioned with respect to the waveguide toprovide control of a coupling coefficient for conversion between onepolarisation mode and another, and control of the relative phase betweenthe two modes; whereby both modes experience the same materialrefractive index, and any phase mismatch between the modes can becorrected by applying a suitable bias voltage through the electrodemeans. This document discloses a waveguide in which three top electrodesare formed on a waveguide core comprising a lithium niobate layer. Acladding layer is provided between the three top electrodes and thewaveguide core. The three top electrodes are configurable to apply ahorizontal electrical field and vertical electrical field in the lithiumniobate. Because a relatively large voltage drop is likely to occur inthe cladding layer when respective voltages are applied to the three topelectrodes, the horizontal and/or vertical electrical fields produced inthe waveguide core by such voltage application are expected to be oflower field strength, specifically in the lithium niobate layer. Also, aground electrode is absent in the present configuration so field leakagemay occur and particularly the vertical electrical field strength may befurther reduced. In respect of if the waveguide core of the presentconfiguration were to comprise barium titanate, the above-discussedfactors may cause a reduced efficiency with which it may be poled and/orits refractive index modulated and, therefore, an overall reducedelectro-optic response is to be expected.

The document titled, “Low power Mach-Zehnder modulator insilicon-organic hybrid technology”, by Palmer et al. published in IEEEphotonics technology letters, vol. 25, no. 13, Jul. 1, 2013, discloses asilicon-organic hybrid modulator based on a Mach-Zehnder interferometer.The device consists of a strip-loaded slot waveguide covered with anelectro-optic polymer cladding. This document discloses a slotwave-guide modulator in which a vertical slot filled with electro-opticactive materials is provided between silicon block electrodes. A singleelectrical field in a single direction is disclosed, there is noperpendicular field component. This may pose a limitation for the use ofthis configuration in a waveguide structure with a core in which theelectro-optic active material is, for example, barium titanate, since itcannot be poled perpendicularly with respect to the modulatingelectrical field and so a reduced electro-optic response is likely.

The document titled, “AlGaAs—GaAs polarisation converter withelecro-optic phase mismatch control”, by Grossard et al. published inIEEE photonics technology letters, vol. 13, no. 8, August, 2001,discloses an electro-optic transverse magnetic-transverse electric modeconverter with phase mismatch control integrated in AlGaAs—GaAs.Voltages applied to the three electrodes facilitate respectivehorizontal and vertical electrical fields in the AlGaAs—GaAs layer. Thisconfiguration may not be suitable for implementing a waveguide structurein which the core comprises high permittivity and/or electro-opticactive materials such as, for example, barium titanate. A relativelylarge voltage drop is likely to occur in the cladding layer between thethree electrodes and the core when respective voltages are applied tothe three electrodes. So, it is likely that the horizontal and verticalelectrical fields produced in the core by such voltage application areof lower field strength. Furthermore, in the absence of a groundelectrode in the present configuration, the vertical electrical fieldstrength is likely to be reduced. In combination, these factors maycontribute to a reduced electro-optic response of this configuration.

In the document titled, “A review of lithium niobate modulators forfiber-optic communications systems”, by Wooten et al. published in IEEEjournal of selected topics in quantum electronics, vol. 6, issue 1,January-February 2000, a status of the lithium niobate externalmodulator technology is reviewed. Other waveguide structures andelectro-optic device/material technology have been disclosed in patentdocuments US2012/0148183A1, US7224878B1, EP1271220B1, US7224869B2,US8244076B2, US2004/0114208A1 and in the documents titled, “A strongelectro-optically active lead-free ferroelectric integrated on silicon”by Abel et al. published in Nature communications 4, Article no. 1671,April 2013, and “BaTiO3-SrTiO3 multilayer thin film electro-opticwaveguide modulator”, by Abel et al. published in Applied PhysicsLetters, vol. 89, issue 24, December 2006.

Accordingly, it is a challenge to provide a waveguide structure, formingthe basis of a silicon-based photonics structure, with an integratedelectro-optic active material, that mitigates and/or obviates thedrawbacks associated with previously-proposed waveguide structures.

SUMMARY OF THE INVENTION

According to an embodiment of a first aspect of the present invention,there is provided a waveguide structure comprising: a core comprising alayer of at least an electro-optic dielectric material, a layer of atleast a semiconductor material provided below the electro-optic materialand a layer of at least a semiconductor material provided above theelectro-optic material, and electrodes that are configurable for voltageapplication, wherein: the electro-optic dielectric material has aPockets tensor containing at least one non-vanishing element rij wherei≠j, and the electrodes comprise respective sets of electrodescomprising a set of electrodes that are provided substantially in directcontact with the electro-optic dielectric material, and a further set ofelectrodes comprising at least an electrode provided substantially indirect contact with the semiconductor material below the electro-opticmaterial and at least an electrode provided substantially in directcontact with the semiconductor material above the electro-opticmaterial, wherein the respective sets of electrodes are configurable toapply in the electro-optic material, when the waveguide structure is inuse, at least a substantially horizontal electrical field and at least asubstantially vertical electrical field that are orientatedsubstantially perpendicular relative to each other.

The electrode configuration in an embodiment of the present inventioncomprises respective sets of electrodes. A set of electrodes areprovided substantially in direct contact with the electro-opticdielectric material. Also provided is a further set of electrodes ofwhich at least an electrode is provided substantially in direct contactwith the semiconductor material above the electro-optic material and atleast an electrode that is provided substantially in direct contact withthe semiconductor material below the electro-optic material. By way ofthe electrode configuration in an embodiment of the present invention,the horizontal electrical field and vertical electrical field areorientated substantially perpendicular to each other, which mayfacilitate an enhanced electro-optic effect and/or electro-opticresponse of the electro-optic material.

The electrode configuration according to an embodiment facilitates theapplication of a relatively higher horizontal electrical field and/orvertical electrical field to the electro-optic material compared topreviously-proposed solutions. This may provide the advantage of a lowerpower consumption since the application of a given electrical field inthe electro-optic material may be done by applying lower voltages to thesets of electrodes in an embodiment of the present invention compared topreviously-proposed solutions.

In contrast to previously-proposed structures and/or devices, there isno cladding layer between the electrodes and the electro-optic materialin an embodiment of the present invention. Thus, higher electrical fieldstrengths of the horizontal electrical field and/or the verticalelectrical field in the electro-optic material are expected. Thisfeature may further enhance the electro-optic effect and/orelectro-optic response of the electro-optic material in an embodiment ofthe present invention. This feature may also contribute toadvantageously further reducing the power consumption of an embodimentof the present invention as discussed hereinabove. The absence ofincorporating a cladding layer reduces the number of fabrication stepsof an embodiment of the present invention compared topreviously-proposed devices and/or structures.

An embodiment of the present invention may be suitable to form the basisof a silicon-based waveguide structure. In this regard, it may benefitfrom the mature fabrication and processing technology based on silicon.Because the electro-optic material may be integrated with ease andwithout the need for special processing steps and/or equipment, such as,for example, spin-coating, in an embodiment of the present invention, itmay be capable of matching the performance capability ofmodulators/optical structures based on electro-optic materials in termsof bandwidth capacity by facilitating higher order modulation schemesand so outperform current silicon-based modulators.

Preferably, the horizontal electrical field and the vertical electricalfield each facilitate a given corresponding effect in the electro-opticmaterial. It is more efficient and effective to use each of thehorizontal electrical field and the vertical electrical field tofacilitate a given corresponding effect in the electro-optic materialthan using a single electrical field for the same purpose. Because thehorizontal electrical and vertical electrical field can be independentlycontrolled, this feature provides the advantage that the givencorresponding effects, which consequently occur in the electro-opticmaterial due to applying the horizontal electrical field and verticalelectrical field thereto, may be facilitated in a desired manner.

Desirably, the horizontal electrical field and the vertical electricalfield are configurable to interchangeably facilitate a given effect inthe electro-optic material. This confers flexibility and versatility toan embodiment of the present invention and extends its use withelectro-optic materials and/or applications where such a feature isdesired.

Preferably, a given set of electrodes of the respective sets ofelectrodes are configurable to pole ferroelectric domains in theelectro-optic material. This feature provides the advantage of increasedsuitability for the use of an embodiment of the present invention withelectro-optic materials such as, for example, barium titanate. Byexploiting the relatively large electro-optic effect of barium titanate,the voltage and/or power consumption requirements of existingsilicon-based optical modulators may be lowered by a factor of >100 withan embodiment of the present invention.

Desirably, a given set of electrodes of the respective sets ofelectrodes are configurable to modify a refractive index of theelectro-optic material. By voltage application to a given set ofelectrodes of the respective sets of electrodes, the refractive index ofthe electro-optic material may be modified by either a verticalelectrical field or a horizontal electrical field in an embodiment ofthe present invention. This feature imparts the advantages offlexibility, ease of operation and control to an embodiment of thepresent invention.

Preferably, an embodiment of the present invention is configurable tomodify at least one of the horizontal electrical field and the verticalelectrical field in response to a given temperature variation. Such acorrection/compensation feature may extend the versatility andperformance of an embodiment of the present invention ahead ofpreviously-proposed waveguide structures and/or devices.

Desirably, an embodiment of the present invention is configurable tomodify at least one of the horizontal electrical field and the verticalelectrical field in response to a given dimension deviation. Such acorrection/compensation feature may further extend the versatility andperformance of an embodiment of the present invention ahead ofpreviously-proposed waveguide structures and/or devices.

Preferably, an embodiment of the present invention is configurable suchthat the horizontal electrical and the vertical electrical field areapplied one of: simultaneously and consecutively to the electro-opticmaterial. This feature extends the advantages of versatility andflexibility to an embodiment of the present invention and increasing itsscope of use in an optical application/structure/device where such afeature may be desired.

Desirably, the respective sets of electrodes are provided onsubstantially a same plane relative to the electro-optic material. Thisfeature may provide the advantage of ease of fabrication and/orintegration of an embodiment of the present invention with respect to anoptical device/system/structure such as, for example, a photonicstructure.

Preferably, the electro-optic material exhibits a Kerr effect in therange of 1e⁻¹⁰ m²/V² to 1e⁻²⁵ m²/V². The electro-optic material ischosen so as to have a dominant Pockets effect compared to itsassociated Kerr effect, this being desirable for certain applicationsfor which an embodiment of the present invention may be suited for.

Desirably, an embodiment of the present invention comprises a slotwaveguide structure. This structure may provide the advantage ofrelatively strong light confinement in the layer of electro-opticmaterial.

Preferably, the electro-optic dielectric material comprises at least oneof barium titanate and barium strontium titanate. In contrast topreviously-proposed waveguide structures, an embodiment of the presentinvention is particularly suitable for use with electro-optic materialssuch as barium titanate and barium strontium titanate, which exhibit alarge electro-optic effect compared to other electro-optic materials. Inrespect of barium titanate, and as discussed hereinabove, it displaysthe strongest electro-optic effect and/or electro-optic response when: apoling electrical field is applied to align its ferroelectric domainsand the modulating electrical field that is applied to vary itsrefractive index is substantially perpendicular to the poling field.These conditions are all met in an embodiment of the present inventionbecause the vertical electrical field and the horizontal electricalfield, which may be used for poling the electro-optic material andmodifying its refractive index, are orientated substantiallyperpendicularly relative to each other in the electro-optic material dueto the electrode configuration feature of an embodiment of the presentinvention. This may facilitate lower power consumption and higher speedperformance of an embodiment of the present invention compared topreviously-proposed waveguide structures.

Desirably, at least one of the semiconductor materials provided aboveand below the electro-optic material comprises one of: a Group IVmaterial, a Group III-V material, a crystalline material, apolycrystalline material and an amorphous material. In this way, thecompatibility of an embodiment of the present invention withwell-established, cost-effective and mass-fabrication processingtechniques is increased.

Preferably, at least one of the semiconductor materials provided aboveand below the electro-optic material comprises amorphous silicon. Thisfeature may confer the advantage of ease of integration of an embodimentof the present invention with silicon photonic devices and/orstructures.

According to an embodiment of a second aspect of the present invention,there is provided an optical structure comprising at least a waveguidestructure according to an embodiment of the first aspect of the presentinvention. A waveguide structure according to an embodiment of the firstaspect of the present invention may also be used to enable activephotonic devices and/or structures such as Mach Zehnderinterferometers/modulators and ring resonators/switches. Theadvantageous properties and/or features of a waveguide structureaccording to an embodiment of the first aspect of the present inventionare imparted to such photonic devices and/or structures.

A corresponding method aspect is also provided, and so, according to anembodiment of a third aspect of the present invention, there is provideda method for fabricating a waveguide structure comprising the steps of:providing a core comprising a layer of at least an electro-opticdielectric material, a layer of at least a semiconductor material belowthe electro-optic material and a layer of at least a semiconductormaterial above the electro-optic material, and providing electrodes thatare configurable for voltage application, wherein: the electro-opticdielectric material is selected to have a Pockels tensor containing atleast one non-vanishing element rij where i≠j, and the electrodes areprovided as comprising respective sets of electrodes comprising a set ofelectrodes that are provided substantially in direct contact with theelectro-optic dielectric material and a set of electrodes comprising atleast an electrode that is provided substantially in direct contact withthe semiconductor material provided below the electro-optic material andat least an electrode that is provided substantially in direct contactwith the semiconductor material provided above the electro-opticmaterial, wherein the respective sets of electrodes are configurable toapply in the electro-optic material, when the waveguide structure is inuse, at least a substantially horizontal electrical field and at least asubstantially vertical electrical field that are orientatedsubstantially perpendicular relative to each other.

Features of one aspect may be applied to another aspect and vice versa.Any of the embodiments shown and/or described may be combined with eachother. This is also possible for one or more features of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1 schematically illustrates an embodiment of the present invention;

FIGS. 2a and 2b respectively schematically illustrate a horizontalelectrical field and a vertical electrical field according to anembodiment of the present invention;

FIGS. 3a and 3b respectively schematically illustrate an embodiment ofthe present invention;

FIGS. 4a and 4b respectively schematically illustrate waveguidestructures used for comparison with an embodiment of the presentinvention as shown in FIGS. 3a and 3 b;

FIGS. 5a and 5b schematically illustrate quantitative calculations ofthe horizontal electrical field and vertical electrical field obtainedwith an embodiment of the present invention as shown in FIGS. 3a and 3band the structures shown in FIGS. 4a and 4 b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENTINVENTION

Within the description, the same reference numerals or signs have beenused to denote the same parts or the like.

Reference is now made to FIG. 1 by way of which the layer/structuralcomposition of the core 2 of a waveguide structure 1 according to anembodiment of the present invention is described. The core 2 comprisesrespective layers of an electro-optic dielectric material 3, asemiconductor material 4 provided below the electro-optic material 3 anda semiconductor material 5 provided above the electro-optic material 3.The semiconductor materials deposited below 4 and above 5 theelectro-optic material 3 are chosen to each have a higher refractiveindex than the electro-optic material 3. A feature 5′ is also providedin conjunction with the semiconductor layer 5 provided above theelectro-optic material 3. It is created by processing the semiconductorlayer 5 and is of the same material composition as semiconductor layer5. Also provided are electrodes 6, 6′, 7, 8, to which voltages can beapplied via corresponding electrical terminals 60, 60′, 7′, 8′. Theelectro-optic dielectric material 3 is characterised in that it has aPockels tensor containing at least one non-vanishing element rij wherei≠j and its largest Pockels coefficient is in the range of 0.01 to100000 pm/V.

The electrode configuration in an embodiment of the present inventioncomprises respective sets of electrodes 6, 6′, 7, 8. A set of electrodes6, 6′ is provided substantially in direct contact with the electro-opticdielectric material 3, such a set of electrodes being hereinafterreferred to as the first set of electrodes. Another set of electrodes 7,8 is also provided comprising at least an electrode 7 providedsubstantially in direct contact with the semiconductor material 4 belowthe electro-optic material 3 and at least an electrode 8 providedsubstantially in direct contact with the semiconductor material 5 abovethe electro-optic material 3, such a set of electrodes being hereinafterreferred to as the second set of electrodes. In a preferred embodimentof the present invention and as can be clearly seen from FIG. 1, thesecond set of electrodes 7, 8 is implemented by a respective pair ofmetallic electrodes 7, 8 placed in direct contact with the semiconductormaterials below 4 and above 5 the electro-optic material 3. In anembodiment of the present invention, the respective sets of electrodes6, 6′, 7, 8 are provided on substantially a same plane relative to theelectro-optic material 3. In this regard, and with reference to FIG. 1,the respective sets of electrodes 6, 6′, 7, 8 are provided relative to atop surface of the electro-optic material 3 onto which the semiconductormaterial 5 is deposited. The core 2 and the respective sets ofelectrodes 6, 6′, 7, 8, are provided on a substrate 10 in an embodimentof the present invention.

With respect to a material composition of an embodiment of the presentinvention as shown in FIG. 1, the waveguide core 2 is provided on asubstrate 10, which is preferably a buried oxide layer and comprisessilicon dioxide. The electro-optic dielectric material 3 is chosen tocomprise barium strontium titanate and most preferably barium titanate.At least one of the respective layers of semiconductor materials 4, 5provided below and above the electro-optic material 3 is chosen tocomprise one of a Group IV material, a Group III-V material, acrystalline material, a polycrystalline material and an amorphousmaterial. In this regard, at least one of the layers of semiconductormaterials 4, 5 provided below and above the electro-optic material 3 ischosen to comprise amorphous silicon. In an embodiment of the presentinvention, the semiconductor layers 4, 5 provided below and above theelectro-optic material 3 may both comprise crystalline silicon or,alternatively, one of them may comprise crystalline silicon and theother may comprise amorphous silicon. A cladding 9 of silicon dioxide isalso provided.

Reference is now made to FIGS. 2a and 2b , which respectivelyschematically illustrate a horizontal electrical field 11 and a verticalelectrical field 12, each of which may be used to facilitate a givencorresponding effect in the electro-optic material 3, in an embodimentof the present invention. This feature is of particular advantage inrespect of the electro-optic material 3 in an embodiment of the presentinvention, which is chosen to have a Pockets tensor with at least onenon-vanishing element rij where i≠j. This is because such acorresponding effect, which may be one of poling the electro-opticmaterial and modulating its refractive index, may be facilitated in thenon-diagonal directions of the crystal structure of the electro-opticmaterial 3.

As can be clearly seen from FIGS. 2a and 2b , the horizontal electricalfield 11 and the vertical electrical field 12 are respectively in-planeand out-of-plane relative to a surface of the electro-optic material 3onto which the respective sets of electrodes 6, 6′, 7, 8 are provided.Because of the electrode configuration 6, 6′, 7, 8 feature of anembodiment of the present invention as hereinabove described, thehorizontal electrical field 11 and the vertical electrical field 12 areorientated substantially perpendicular to each other in theelectro-optic material 3.

Reference is now made to FIG. 2a in order to describe how the horizontalelectrical field 11 is facilitated in an embodiment of the presentinvention. Applying respective voltages to the electrical terminals 60,60′ corresponding to the first set of electrodes 6, 6′ causes a voltagedrop to occur horizontally in the electro-optic material 3 spanningbetween the first set of electrodes 6, 6′. In this way, and as can beclearly seen from FIG. 2a , a horizontal electrical field 11 ishomogeneously provided substantially in the full extent of theelectro-optic material 3 of the waveguide core 2. Accordingly, theelectrode configuration of an embodiment may facilitate the applicationof a relatively higher horizontal electrical field 11 to theelectro-optic material 3 than previously-proposed solutions.

Reference is now made to FIG. 2b in order to describe how the verticalelectrical field 12 is facilitated in an embodiment of the presentinvention. The electrodes 7, 8 of the second set of electrodes 7, 8 areimplemented using metallic electrodes/pads contacting the respectivelayers of semiconductor materials 4, 5 provided below and above theelectro-optic material 3. By way of this arrangement, the electricalpotentials in the respective layers 4, 5 of semiconductor materialsprovided below and above the electro-optic material 3 are substantiallythe same as the electrical potentials in the metallic electrodes 7, 8 incontact thereto. The electrical potentials in the metallic electrodes 7,8 are facilitated by the application of respective voltages to theelectrical terminals 7′, 8′ corresponding thereto. Thus, the layers ofsemiconductor materials 4, 5 provided above and below the electro-opticmaterial 3 perform as semiconducting electrodes and provide acontinuation of the respective metallic electrodes 7, 8 in the regionswhere they respectively contact the electro-optic material 3.Application of respective voltages to the second set of electrodes 7, 8causes a voltage drop vertically and substantially uniformly across theelectro-optic material 3. In this way, the vertical electrical field 12is applied to the electro-optic material 3 in an embodiment of thepresent invention. With respect to the layers of respectivesemiconductor material 4, 5 provided below and above the electro-opticmaterial 3 performing as semiconducting electrodes, the layer ofsemiconductor material 4 provided below the electro-optic material 3performs as a ground electrode and provides the advantage of reducedleakage of the vertical electrical field 12 in an embodiment of thepresent invention. Accordingly, the electrode configuration of anembodiment may facilitate the application of a relatively highervertical electrical field 12 to the electro-optic material 3 thanpreviously-proposed solutions. This feature may also further contributeto lowering the power consumption of an embodiment of the presentinvention as discussed hereinabove.

Other features of an embodiment of the present invention also facilitatea relatively higher horizontal electrical field 11 and/or verticalelectrical field 12 to be applied to the electro-optic material 3. Inthis regard, and in contrast to previously-proposed solutions, there isno cladding layer 9 between the respective electrodes 6, 6′, 7, 8 andthe electro-optic material 3 in an embodiment of the present invention.Thus, higher electrical field strengths of the horizontal electricalfield 11 and/or the vertical electrical field 12 are expected.

An embodiment of the present invention is configured in a slot waveguidestructure, namely, a layer of the electro-optic material 3 is providedbetween two layers of semiconductor materials 4, 5. The electro-opticmaterial 3 is chosen to have a lower refractive index than either of thesemiconductor materials 4, 5 provided in relation thereto. Thisincreases the confinement of light in the electro-optic material 3.

An embodiment of the present invention is particularly suited for usewith barium titanate, which displays the strongest electro-optic effectand/or electro-optic response when: a poling electrical field is appliedto align its ferroelectric domains and a modulating electrical fieldthat is applied to vary its refractive index is substantiallyperpendicular to the poling field. These conditions are all met in awaveguide structure 1 because the horizontal electrical field 11 and thevertical electrical field 12 are orientated substantiallyperpendicularly relative to each other in the electro-optic material 3due to the electrode configuration 6, 6′, 7, 8 feature of an embodimentof the present invention. The horizontal electrical field 11 and thevertical electrical field 12 can each be applied to facilitate the givencorresponding effects of poling the electro-optic material 3 andmodifying its refractive index, which is more effective and efficientthan using a single electrical field to achieve the same purposes,particularly for barium titanate with a large r42 coefficient. That thehorizontal electrical field 11 and the vertical electrical field 12 maybe interchangeably used for providing a given effect in theelectro-optic material 3 is of particular advantage in an embodiment ofthe present invention. In this respect, the horizontal electrical field11 and the vertical electrical field 12 may be interchangeably used forproviding the respective effects of poling the electro-optic material 3and modifying its refractive index when the crystalline orientation ofthe electro-optic material 3 determines whether poling thereof should bedone via a horizontal electrical field 11 or vertical electrical field12—this may be the case for when the electro-optic material 3 comprisesbarium titanate, for example. Where the horizontal electrical field 11is used for poling the electro-optic material 3, and because it issubstantially homogeneously provided in the waveguide core 2 in anembodiment of the present invention as can be ascertained from thediscussion corresponding to FIG. 2a , the electro-optic material 3 maybe poled more effectively and to a larger extent than inpreviously-proposed waveguide structures. So, the electro-optic effectand/or electro-optic response of the electro-optic material 3 in anembodiment of the present invention may be further enhanced compared topreviously-proposed waveguide structures.

As mentioned hereinabove, the horizontal electrical 11 and the verticalelectrical field 12 may be applied simultaneously or consecutively tothe electro-optic material 3. Where they are applied simultaneously anddepending on their respective magnitudes, an arbitrary direction of anelectrical field, which is essentially the sum of the horizontalelectrical field 11 and the vertical electrical field 12, may beobtained in the electro-optic material 3 of the waveguide core 2. Anon-horizontal/vertical electrical field affects different coefficientsof the Pockets tensor and so this feature of an embodiment of thepresent invention may be used to advantage for certain materials and/orapplications. Where the horizontal electrical field 11 and the verticalelectrical field 12 are applied consecutively, one of them can be usedto pole the electro-optic material 3 before operation of a modulator,for example, whilst the other is then modulated at high frequency duringdevice operation.

An embodiment of the present invention is configurable to modify atleast one of the horizontal electrical field 11 and the verticalelectrical field 12 in response to a given temperature variation. Theoptical properties of photonic structures may be changed by theoccurrence of temperature changes/drifts. The refractive index ofelectro-optic materials integrated in such photonic structures may bemodified undesirably due to such temperature changes. In this regard, anembodiment of the present invention confers the advantage that a changein a given optical property, for example, a refractive index of theelectro-optic material 3, due to a given temperature variation may becompensated for by modifying the horizontal electrical field 11 and/orthe vertical electrical field 12 via voltage application to a given setof electrodes 6, 6′, 7, 8 of the respective sets of electrodes 6, 6′, 7,8.

Another feature of an embodiment of the present invention is that it isconfigurable to modify at least one of the horizontal electrical field11 and the vertical electrical field 12 in response to a given dimensiondeviation. In this regard, it is known that design targets may bedeviated from during the fabrication of optical/photonic structures.Even relatively small deviations from given fabrication tolerances maysignificantly and, often undesirably, change the optical properties ofsuch optical/photonic structures. In this respect, an embodiment of thepresent invention may confer the advantage of compensating for changesin given optical properties due to fabrication deviations by modifyingat least one of the horizontal electrical field 11 and the verticalelectrical field 12 via voltage application to a given set of electrodes6, 6′, 7, 8 of the respective sets of electrodes 6, 6′, 7, 8. By way ofexample, a shift in the resonance wavelength of a photonic cavity occurswhen the cavity length is even slightly off its associated designtarget. By modifying at least one of the horizontal electrical field 11and the vertical electrical field 12 in an embodiment of the presentinvention, the refractive index of the electro-optic material 3 may bechanged, thereby correcting for the change in the cavity length causedby the fabrication deviation. This feature extends the advantage ofcompensating and/or correcting for the effects of fabricationuncertainty encountered with respect to an embodiment of the presentinvention and/or any structure that it may integrated into.

Reference is now made to FIGS. 4a and 4b that respectively schematicallyillustrate waveguide structures for comparison with an embodiment of thepresent invention as shown in FIGS. 3a and 3b . The waveguide structureshown in FIG. 4a differs from an embodiment of the present inventionshown in FIGS. 3a and 3b in that its electrodes corresponding to thefirst set of electrodes 6, 6′ in an embodiment of the present inventionare provided on a cladding spacer 9 instead of an electro-optic material3, the latter being provided as directly underlying feature 5′ and abovethe semiconductor material indicated by reference numeral 4. Also, a topelectrode 13 comprising a metallic pad with corresponding electricalterminal 13′ is provided separated by cladding 9 from the rest of thestructural features of the waveguide structure. Turning to the waveguidestructure shown in FIG. 4b , it differs from that shown in FIG. 4a inthat its electrodes 6, 6′ corresponding to, and like, the first set ofelectrodes 6, 6′ in an embodiment are provided on an electro-opticmaterial 3 spanning the underlying semiconductor material 4 rather thana cladding spacer layer 9.

The material composition of the waveguide structure according to anembodiment of the present invention as shown in FIGS. 3a and 3b is asfollows: the waveguide core 2 is provided on a substrate 10, which ispreferably a buried oxide layer and comprises silicon dioxide; theelectro-optic dielectric material 3 comprises barium titanate; therespective layers of semiconductor materials 4, 5 provided below andabove the electro-optic material 3 are both chosen to comprise silicon;the cladding 9 comprises silicon dioxide and the respective sets ofelectrodes 6, 6′, 7, 8 are implemented by way of metallic pads.Corresponding structural features of the comparison waveguide structuresshown in FIGS. 4a and 4b and the waveguide structure according to anembodiment of the present invention shown in FIGS. 3a and 3b have thesame material composition as described hereinabove.

In the waveguide structure of FIG. 3a , according to an embodiment ofthe present invention, a horizontal electrical field 11 is generated inthe electro-optic material, barium titanate layer 3 by voltageapplication to the first set of electrodes 6, 6′. In the waveguidestructure of FIG. 4a , a horizontal electrical field 11′ is generated byvoltage application to the electrodes thereof corresponding to the firstset of electrodes 6, 6′ in FIG. 3a . FIG. 5a schematically illustratesprofiles 14, 15 of the respective horizontal electrical fields 11, 11′that are generated in the x-direction.

As can be clearly seen from FIG. 5a , the horizontal electrical field11′ depicted by profile 15 substantially in the electro-optic material,barium titanate layer 3 in the waveguide core, −250 nm<x<250 nm for theapproximately 500 nm wide waveguide, is of a significantly lowermagnitude for the comparison waveguide structure of FIG. 4a compared tothe horizontal electrical field 11 depicted by profile 14 generated withthe waveguide structure 1 according to an embodiment of the presentinvention as shown in FIG. 3a . This is particularly evident in theregion of the centre of the respective waveguide structures, underlyingfeature 5′. Such an observation in respect of the comparison waveguidestructure of FIG. 4a may be attributed to the cladding spacer layer 9underlying the electrodes used in the generation of the horizontalelectrical field 11′ rather than the electro-optic material, bariumtitanate layer 3 as is the case in an embodiment of the presentinvention as shown in FIG. 3 a.

In the waveguide structure of FIG. 3b , according to an embodiment ofthe present invention, a vertical electrical field 12 is generated inthe electro-optic material 3 comprising barium titanate by voltageapplication to the pairs of electrodes of the second set of electrodes7, 8 as has already been described hereinabove. In the comparisonwaveguide structure of FIG. 4b , a vertical electrical field 12′ isgenerated by voltage application to the pair of out-of-plane electrodes7, which are provided in contact with the silicon layer 4 depositedbelow the electro-optic material, barium titanate layer 3, and to thetop electrode 13. FIG. 5b schematically illustrates profiles 16, 17 ofthe respective vertical electrical fields 12, 12′ that are generated.

In respect of the comparison waveguide structure of FIG. 4b , arelatively large drop of the vertical field 12′ occurs in the cladding 9below the top electrode 13, which separates it from the other structuralfeatures of the comparison waveguide structure, and is accordinglyreduced in the electro-optic material, barium titanate layer 3. Withreference to the waveguide structure 1 according to an embodiment of thepresent invention as shown in FIG. 3b , and in the absence of a claddingspacer 9 between the second set of electrodes 7, 8 and theelectro-optic, barium titanate layer 3, the magnitude of the verticalelectrical field 12 in the electro-optic, barium titanate layer 3 ishigher than what is obtained with the comparison waveguide structure ofFIG. 4b . Accordingly, the comparison waveguide structure of FIG. 4b hasa lower associated electro-optic response, which would lead to higherpower consumption than in the case of the waveguide structure 1embodying the present invention and as shown in FIG. 3 b.

From FIG. 5b , it can be seen from profile 16 that the verticalelectrical field 12 in a waveguide structure 1 according to anembodiment of the present invention as shown in FIG. 3b begins toincrease at the interface between the buried oxide substrate layer 10and the silicon layer 4 provided below the electro-optic, bariumtitanate layer 3, corresponding to y=−500 nm in FIG. 5b . The verticalelectrical field 12 is of highest magnitude in the electro-optic, bariumtitanate layer 3 between its respective interfaces with the siliconlayers provided below 4 and above 5 in relation thereto. In contrast,and referring to profile 17, the magnitude of the vertical electricalfield 12′ generated by the comparison waveguide structure of FIG. 4b ismuch smaller in the electro-optic, barium titanate layer 3 in suchcorresponding regions.

An embodiment of the present invention is not restricted to theelectro-optic material 3 comprising barium titanate. Any other suitableelectro-optic material, which has a high-permittivity and/or in whichthe application of a horizontal electrical field 11 and a verticalelectrical field 12 that are orientated substantially perpendicular toeach other is desired may be used.

The second set of electrodes 7, 8 provided on the respectivesemiconductor materials 4, 5 below and above the electro-optic material3 are not restricted to an arrangement of pairs of electrodes 7, 8symmetrically provided relative to the waveguide centre. A scenariowhere an electrode 7, 8 is provided on each of the semiconductormaterials 4, 5 deposited below and above the electro-optic material 3 isalso encompassed within the scope of an embodiment of the presentinvention.

Regarding the electrode arrangement in an embodiment of the presentinvention, the order of the electrodes from the waveguide core 2 to thesides of the waveguide 1 may be interchanged. For example, the metallicelectrodes 7 contacting the semiconductor material 4 deposited under theelectro-optic material 3 may be placed closer to the waveguide core 2than the first set of electrodes 6, 6′.

The present invention has been described purely by way of example andmodifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and where appropriate, theclaims and the drawings may be provided independently or in anyappropriate combination.

The invention claimed is:
 1. A waveguide structure (1) comprising: acore (2) comprising a layer of at least an electro-optic dielectricmaterial (3), a layer of at least a semiconductor material (4) providedbelow the electro-optic material (3) and a layer of at least asemiconductor material (5) provided above the electro-optic material(3), and electrodes (6, 6′, 7, 8), that are configurable for voltageapplication, wherein: the electro-optic dielectric material (3) has aPockels tensor containing at least one non-vanishing element rij wherei≠j, and the electrodes (6, 6′, 7, 8) comprise respective sets ofelectrodes (6, 6′, 7, 8) comprising a set of electrodes (6, 6′) that areprovided substantially in direct contact with the electro-opticdielectric material (3), and a further set of electrodes (7, 8)comprising at least an electrode (7) provided substantially in directcontact with the semiconductor material (4) below the electro-opticmaterial (3) and at least an electrode (8) provided substantially indirect contact with the semiconductor material (5) above theelectro-optic material (3), wherein the respective sets of electrodes(6, 6′, 7, 8) are configurable to apply in the electro-optic material(3), when the waveguide structure (1) is in use, at least asubstantially horizontal electrical field (11) and at least asubstantially vertical electrical field (12) that are orientatedsubstantially perpendicular relative to each other.
 2. A waveguidestructure (1) as claimed in claim 1 wherein the horizontal electricalfield (11) and the vertical electrical field (12) are each configurableto facilitate a given corresponding effect in the electro-optic material(3).
 3. A waveguide structure (1) as claimed in claim 1 wherein thehorizontal electrical field (11) and the vertical electrical field (12)are configurable to interchangeably facilitate a given effect in theelectro-optic material (3).
 4. A waveguide structure (1) as claimed inclaim 1, wherein a given set of electrodes of the respective sets ofelectrodes (6, 6′, 7, 8) are configurable to pole ferroelectric domainsin the electro-optic material (3).
 5. A waveguide structure (1) asclaimed in claim 1 wherein a given set of electrodes of the respectivesets of electrodes (6, 6′, 7, 8) are configurable to modify a refractiveindex of the electro-optic material (3).
 6. A waveguide structure (1) asclaimed in claim 1 configurable to modify at least one of the horizontalelectrical field (11) and the vertical electrical field (12) in responseto a given temperature variation.
 7. A waveguide structure (1) asclaimed in claim 1 configurable to modify at least one of the horizontalelectrical field (11) and the vertical electrical field (12) in responseto a given dimension deviation.
 8. A waveguide structure (1) as claimedin claim 1 configurable such that the horizontal electrical (11) and thevertical electrical field (12) are applied one of: simultaneously andconsecutively to the electro-optic material (3).
 9. A waveguidestructure (1) as claimed in claim 1 wherein the respective sets ofelectrodes (6, 6′, 7, 8) are provided on substantially a same planerelative to the electro-optic material (3).
 10. A waveguide structure(1) as claimed in claim 1 wherein the electro-optic material (3)exhibits a Kerr effect in the range of 1e⁻¹⁰ m₂/V² to 1e⁻²⁵ m²/V².
 11. Awaveguide structure (1) as claimed in claim 1 comprising a slotwaveguide structure (3, 4, 5, 5′).
 12. A waveguide structure (1) asclaimed in claim 1 wherein the electro-optic dielectric material (3)comprises at least one of barium titanate and barium strontium titanate.13. A waveguide structure (1) as claimed in claim 1 wherein at least oneof the semiconductor materials (4, 5) provided above and below theelectro-optic material (3) comprises one of: a Group IV material, aGroup III-V material, a crystalline material, a polycrystalline materialand an amorphous material.
 14. A waveguide structure (1) as claimed inclaim 1 wherein at least one of the semiconductor materials (4, 5)provided above and below the electro-optic material (3) comprisesamorphous silicon.
 15. A method for fabricating a waveguide structure(1) comprising the steps of: providing a core (2) comprising a layer ofat least an electro-optic dielectric material (3), a layer of at least asemiconductor material (4) below the electro-optic material (3) and alayer of at least a semiconductor material (5) above the electro-opticmaterial (3), and providing electrodes (6, 6′, 7, 8) that areconfigurable for voltage application, wherein: the electro-opticdielectric material (3) is selected to have a Pockels tensor containingat least one non-vanishing element rij where i≠j, and the electrodes (6,6′, 7, 8) are provided as comprising respective sets of electrodes (6,6′, 7, 8) that comprise a set of electrodes (6) that are providedsubstantially in direct contact with the electro-optic dielectricmaterial (3) and a set of electrodes (7, 8) comprising at least anelectrode (7) that is provided substantially in direct contact with thesemiconductor material (4) provided below the electro-optic material (3)and at least an electrode (8) that is provided substantially in directcontact with the semiconductor material (5) provided above theelectro-optic material (3), wherein the respective sets of electrodes(6, 6′, 7, 8) are configurable to apply in the electro-optic material(3), when the waveguide structure (1) is in use, at least asubstantially horizontal electrical field (11) and at least asubstantially vertical electrical field (12) that are orientatedsubstantially perpendicular relative to each other.